Photoelectrophoretic imaging system employing preliminary electrophoretic disposition of the imaging suspension

ABSTRACT

An electrophoretic imaging process is disclosed whereby an useable positive image may be directly produced from negative input. The light-sensitive imaging suspension is electrophoretically introduced into the electrode configuration prior to light exposure in the presence of an electric field.

Unlted States Patent 1111 3, 20,94

[72] Inventor ArthurWalsh I501 FieldofSearch.... 204/181. Rochester.N.Y.180; 96/1, 1.2. 1.3, 1.4

21 Appl. No. 764,718

122] Filed Oct. 3, 1968 References Cited [45] Patented Nov. I6, I97]UNITED STATES PATENTS [73] Assignee Xerox Corporation 3,384.566 5/1968Clark 204/181 Rochester-P1X- 3.427.242 2/1900 Mihaylor 204/300 [54]PHOTOELECTROPHORETIC IMAGING SYSTEM EMPLOYING PRELIMINARYELECTROPHORETIC DISPOSITION OF THE IMAGING SUSPENSION 14 Claims, 2Drawing Figs.

[52] U.S.C1.... 204/181.

06/1 Rios/1 2.66/1111150/14, 204/180 51 Int. Cl ..c03 13/22 Prim aryExaminer-George F. Lesmes Assisi/m! Examiner-John C. Cooper, 111

Attorneys-James J. Raiabate, David C. Petre and Barry .11

Kesselman ABSTRACT: An electrophoretic imaging process is disclosedwhereby an useable positive image may be directly produced from negativeinput. The light-sensitive imaging suspension is electrophoreticaflyintroduced into the electrode configuration prior to light exposure inthe presence of an e1ectric field.

PATENTEBuuv 16 I97! FIG.

INVENTOR.

ARTHUR WALSH ATTO EV PHOTOELECTROPHORETIC IMAGING SYSTEM EMPLOYINGPRELIMINARY ELECTROPIIORETIC DISPOSITION OF THE IMAGING SUSPENSIONBACKGROUND OF THE INVENTION This invention relates to an imaging systemand more specifically to an electrophoretic imaging system.

In photoelectrophoretic imaging, colored photosensitive particles aresuspended in an insulating carrier liquid. This suspension is placedbetween a pair of electrodes, subjected to a potential difference andexposed to an image to be reproduced. Ordinarily, in carrying out theprocess, the imaging suspension is placed on a transparent electricallyconductive transparent plate in the fonn of a thin film and exposure ismade through the transparent plate while a second generallycylindrically shaped electrode is rolled across the surface of thesuspension. The particles are believed to hear an initial charge whensuspended in the liquid carrier which causes them to be attracted to thetransparent base electrode and, upon exposure, to change polarities byexchanging charge with the base electrode such that the exposedparticles will migrate away from the base electrode to the rollerelectrode thereby forming images on both of the electrodes by particlesubtraction, each image being complementary one to the other. Theprocess may be used to produce both polychromatic and monochromaticimages. In the latter instance a single color photoresponsive particlemay be used in the suspension or a number of differently coloredphotoresponsive particles may be used in the suspension all of whichrespond to the same wavelength of light exposure. An extensive anddetailed description of the photoelectrophoretic imaging techniques suchas described above can be found in U.S. Pat. Nos. 3,384,565 and3,384,566.

In the case of the polychromatic imaging process the imaging suspensionwill contain a plurality of at least two differently colored particlesin a carrier liquid, each of said particles comprising an electricallyphotosensitive pigment whose principle light absorption bandsubstantially coincides with its principle photosensitive response.Thus, the pigment represents both theprimary electrically photosensitiveingredient and the primary colorant for the particular particle insuspension. The particles in the polychromatic system should preferablyhave intense pure colors and be highly photosensitive. It is preferredthat the particles migrate with minimum exposure to activatingelectromagnetic radiation and that particles of each color migrate to anequal extent upon equal exposure to light of their complementary color.Where the particle mix is exposed to a multicolored image, particlesmigrate in proportion to the intensity of the light which they absorb.This migration should take place with a minimum of electricalinteraction between particles of different colors. Thus, it is preferredand desired that particles selectively remain on one of the electrodesin image configuration with unwanted particles migrating to the otherelectrode in the system. For exam ple, when a mixture comprising cyan,magenta and yellow particles is exposed to an image by yellow light, thecyan and magenta particles should migrate thus leaving behind an imagemade up of yellow particles. Similarly, when exposed to a multicoloredimage, different colored particles absorb light of their complementarycolor in appropriate image area and migrate leaving a full colored imagecorresponding to the original.

Although it has generally been found that good quality images can beproduced, especially when a relatively insulating blocking" surface isused as a part of the roller electrode in the system, due to the natureof the imaging suspension the image formed on the surface of the rollerelectrode not only contains pigment particles which have migrated to therespective surface in response to the impinging light source, but, inaddition, due to the bipolarity properties of the pigments utilized inthe system, also contains a goodly number of image degrading chargedpigment particles which inherently migrate to the roller electrodesurface during the imaging-exposure phase of the process therebycontaminating the image formed on the roller electrode and making itunuseable. Thus, it has generally been necessary to form the desiredimage on the surface of the transparent electrode and then transfer thisimage to the surface of a receiving sheet. Thus, an optically and colornegative input produces a useable optically and color negative image onthe transparent electrode and an optically and color positive imageproduces a useable optically and color positive image on the transparentelectrode. Therefore, the process as it is presently known has beenfound generally unsuitable for the formation, in a one-step system, of apositive image utilizing a color negative transparency as the input.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide an imaging system which will overcome the above-noteddisadvantages.

It is a further object of this invention to provide a highqualityelectrophoretic imaging system.

Yet still a further object of this invention is to provide aneleetrophoretic imaging system capable of producing an optically andcolor positive image directly from an optically and color negativeinput.

Another object of this invention is to provide an improved one-steppolychromatic imaging system.

The foregoing objects and others are accomplished in accordance with thepresent invention generally speaking by providing an imaging suspensionof photoelectrophoretic imaging particles in an insulating carrierliquid. The imaging suspension is electrophoretically deposited onto thesurface of an electrode which is at least partially transparent.Following the deposition of the imaging suspension onto the surface ofthe above-mentioned transparent electrode, the suspension is exposedselectively to an electromagnetic radiation source through thetransparent electrode as an imaging roller is passed across the surfaceof the imaging suspension with a potential being applied across theimaging suspension concurrent with the exposure step. The imaging stepmaybe carried out with a negative transparency being projected onto theimaging suspension by way of the transparent electrode. The pigmentimage formed on the surface of the imaging roller is the complementarycolor of the light exposing the imaging suspension and therefore animage optically reversed from the input image is obtained directly.

Direct image color reversal has been lacking in the electrophoreticimaging systems presently known inasmuch as a number of undesirablepigment particles migrate to the surface of the imaging electrode alongwith the desired pigment particles to the extent of completely obscuringthe final image. It has been determined in the course of the presentinvention that by initially applying the imaging suspensionelectrophoretically to the surface of the transparent electrode utilizedin the system the unwanted particles are removed from the suspension. Itis hypothesized that the imaging suspension is electrophoretically splitto remove the above-mentioned image degrading particles which heretonowprevented the acceptable reproduction in a direct one step process of apositive image from a negative input or, vice versa, an optical or colornegative from an optically positive color image input. Obviously, inproducing a positive image directly on the roller electrode surfacethere is simultaneously produced a negative image on the surface of thetransparent electrode. In other words the image on the transparentelectrode optically matches the input while a usable image on theimaging electrode is color reversed or negative relative to the input.The viewing sense of the formed images may be right-reading or a mirrorimage depending from which side of the transparent electrode the imageis viewed or whether or not a mirror is placed in the projection system.For example, the image viewed on the suspension side of the transparentelectrode of Fig. 2 is wrong-reading but optically a color positive ofthe input projected. The image formed on the other electrode will beright-reading and a color negative of the input projected. To make bothright-reading, peel off the surface of the imaging electrode with itsimage on it, and transfer the image on the transparent electrode to atransport sheet.

The electrophoretic imaging system of the present invention employsintensely colored pigment particles which serve both as the colorant andas the photosensitive material which apparently undergoes a net changein charge polarity upon exposure to activating radiation by interactionwith one of the electrodes in the system. No additional photosensitiveelements or materials are necessary thus providing a very simple andinexpensive imaging technique. In a monochromatic system a singlecolored photoresponsive particle may be used in the suspension of anumber of differently colored photoresponsive particles may be used inthe suspension all of which respond to the same wavelength of lightexposure. As a result of a mixture of two or more differently coloredparticles each of which is sensitive only to light of a specificwavelength, the images are produced in a variety of colors. Theparticles respond in the regions of the spectrum of their principlelight absorption with the cyan, magenta and yellow particles respondingto red, green and blue fight-respectively. Thus this system is mostsuited to subtractive c'olor synthesis.

BRIEF DESCRIPTION OF DRAWINGS The invention is further illustrated inthe accompanying drawings wherein:

FIG. I represents a side sectional view of the initial phase of theprocess of the present invention whereby the imaging suspension iselectrophoretically deposited onto the surface of the transparentelectrode; and

FIG. 2 represents a simple exemplary system for carrying out the imagingphase of the process herein described.

DETAILED DESCRIPTION Referring now to FIG. I there is seen a transparentelectrode generally designated 1 which in this exemplary instance ismade up of a layer of optically transparent glass 2 overcoated with athin transparent layer 3 of tin oxide. This configuration iscommercially available under the trade name NESA glass This electrodewill hereinafter be referred to as the injecting electrode. A rollerelectrode 4, having coated on its surface a thin layer 6 of the imagingsuspension of the present invention, is passed across the surface of theinjecting electrode 1, the imaging suspension consisting of finelydivided photosensitive particles dispersed in an insulating carrierliquid. During the description of the invention, the termphotosensitive" when used in reference to the particles in suspensionmay be thought of as being any particle which, once attracted to theinjecting electrode will migrate away from it under the influence of anapplied electric field when it is exposed to actinic electromagneticradiation. The imaging suspension may also contain a sensitizer and/orbinder for the pigment particles which is at least partially soluble inthe suspending or carrier liquid. The roller electrode 4, comprising aconductive central core and blocking layer 9, is connected to a powersource 7 which in turn is connected to the injecting electrode 1 so thatwhen switch 8 is closed an electric field is applied between electrodes4 and l as the blocking electrode passes across the surface of theinjecting electrode. With the potential of the roller 4 being fixed at aone polarity and the corresponding potential of the injecting electrodeat the opposite polarity the imaging suspension 6 is electrophoreticallydeposited 6a, in the absence of light, onto the surface of thetransparent electrode 1. Following the electrophoretic deposition of theimaging suspension on the surface of the injecting electrode the rollerelectrode 4 is replaced in the system by a second roller electrode as isrepresented in FIG. 2. The injecting electrode 1 of FIG. 2 now hascoated on its surface a thin layer 6a of the imaging suspensionelectrophoretically deposited in the manner discussed in FIG. 1. Abovethe liquid imaging suspension 60 is passed an imaging roller electrode10 which in this illustration is represented as a roller having aconductive central core 11 connected to power source 7. The core in thisinstance is covered with a layer of blocking electrode material 12. Theopposite side of potential source 7 is connected to the injectingelectrode as in the manner discussed in FIG. I. Electrode I0 is alsoreferred to as the blocking electrode. The pigment suspension is exposedby way of the projection mechanism made of light source 15, a colornegative transparency to, and a lens system 17. A potential is appliedacross the imaging and injecting electrodes upon the closing of switch8. The imaging electrode 10 having a cylindrical configuration is rolledacross the upper surface of the injecting electrode 1 supporting theimaging suspension 60 with switch 8 being closed during the period ofimage exposure. The light exposure causes exposed particles originallyattracted to the injecting electrode 1 to migrate through the liquidcarrier and adhere to the surface of the blocking electrode material 12to produce an image complementary to that of the original input. Theresulting thereat optically and color reversed image produced on thesurface of the imaging electrode may then be fixed in place such as byplacing a lamination over its surface or by virtue of a dissolved bindermaterial initially present in the carrier liquid, such as a paraffin waxor other suitable binder, the latter coming out of solution as theliquid carrier evaporates. The blocking electrode layer I2 supportingthe optically and color reversed image may then be unfastened andremoved from the roller core 11 to be used as the final print. Thus, theprocess provides a direct one step system for producing optically andcolor reversed images from the input image, in this instance amulticolor print.

When used in the course of the present invention the term injectingelectrode should be understood to mean that it is an electrode whichwill preferably be capable of exchanging charge with the photosensitiveparticles of the imaging suspension when the suspension is exposed tolight so as to allow for a net change in the charge polarity of theparticles. By the term blocking electrode is meant one which is capableof injecting electrons into and receiving electrons from theabove-mentioned photosensitive particles at a negligible rate when theparticles come into contact with the surface of the electrode.Obviously, if all the polarities in this system are reversed thefunction of the electrodes will also be reversed.

It is preferred that the injecting electrode be composed of an opticallytransparent material such as glass overcated with a conductive materialsuch as tin oxide, copper, copper iodide, gold or the like material;however, other suitable materials including many semiconductor materialssuch as raw cellophane, which are ordinarily not thought of asconductors but which are still capable of accepting injected chargecarriers of the proper polarity under the influence of the appliedfield, may be used within the course of the present invention. The useof more conductive materials, however, allows for a cleaner chargeseparation and prevents possible charge buildup on the electrode whichwould tend to diminish the interior electrode field. The blockingelectrode on the other hand is selected so as to prevent or greatlyretard the injection of electrons into the photosensitive pigmentparticles when the particles reach the surface of this electrode. Theblocking electrode base generally will consist of a material which isfairly high in electrical conductivity. Typical conductive materials areconductive rubber and metal foils such as steel, aluminum, copper andbrass. Preferably the core of the blocking electrode will have a highelectrical conductivity in order to establish the desired polaritydifferential. However, if a lowconductivity material is used a separateelectrical connection may be made to the back of the blocking layer ofthe electrode. Although a blocking electrode need not necessarily beused in this system the use of such a layer is preferred because of themarkedly improved results which it is capable of producing. It ispreferred that the blocking layer when used be an insulator or asemiconductor which will not allow for the passage of sufficient chargecarriers under the influence of an applied field so as to discharge theparticles bound to its surface, thereby preventing particle oscillationwithin the system. Although the blocking electrode does allow forpassage of some charge carriers it would still be considered to comewithin the class of preferred materials if it does not allow for thepassage of sufficient charge carrier to recharge the particles to theopposite polarity. Exemplary of the preferred blocking layer materialused are baryta paper, which consists of a paper coated with bariumsulfate suspended in a gelatin solution, Tedlar, a polyvinyl fluoride,and polyurethane. Any other suitable material having a resistivity offrom about lohm-cm. or greater may be employed as the blocking electrodematerial. Typical materials in this resistivity range include celluloseacetate coated papers, polystyrene, polytetrafluoroethylene, andpolyehyleneterephthalate. The baryta paper, Tedlar and the othermaterials used as the blocking layer may be wetted on the back surfacewith tap water or coated with an electrically conductive material. Theblocking electrode layer, when utilized, may be separate replaceablelayer which is either taped to the blocking electrode core or held by asuitable device such as mechanical fasteners which are capable of simplyholding the layer on the electrode. In the alternative, the layer may bean integral part of the electrode itself, being either adhesivelybonded, laminated, spray coated or otherwise applied to the surface ofthe electrode core. In the case of the present invention utilization ofa separate replaceable layer is preferred.

Any suitable insulating carrier liquid may be used in the course of thepresent invention. Typical materials include decane, dodecane, andtetradecane, molten paraffin wax, molten beeswax and other moltenthermoplastic materials, Sohio Odorless Solvent, a kerosene fractioncommercially available from Standard Oil Company of Ohio and lsopar G, along chain saturated aliphatic hydrocarbon commercially available fromthe Humble Oil Company of New Jersey and mixtures thereof.

A wide range of voltages may be applied between the electrodes in thesystem. For good image resolution, high image density and low backgroundit is preferred that the potential applied be such as to create anelectric field of at least about 300 volts across the imagingsuspension. The applied potential necessary to attain this field ofstrength will, of course, vary depending upon the interelectrode gap andupon the thickness and type of blocking material used on the blockingelectrode surface. For the very highest image qualities the optimumfield is at least about 5,000 volts. The upper limit of field strengthis limited only by the breakdown potential of the suspension andblocking material.

Other configurations may be used similar to that disclosed by theillustrations, and the roller configuration herein represented should beunderstood as merely being illustrative of the present invention. Forexample, the roller-type electrode could be represented in the form of atractor device.

in the polychromatic system, the particles are selected so that those ofdifferent colors respond to different wavelengths in the visiblespectrum corresponding to their principle absorption and further so thattheir spectral response curves do not have substantial overlap, thusallowing for color separation and subtractive multicolor imageformation. Several different particles are employed namely a cyancolored particle sensitive mainly to red light, a magenta coloredparticle sensitive mainly to green light and a yellow colored particlesensitive mainly to blue light. While this is the simplest combination,additional particles having different absorption maxima may be added toimprove color synthesis. When mixed together in the carrier liquid,these particles produced a substantially black liquid and when one ormore of the particles are caused to migrate from the injecting electrodetowards the blocking electrode they leave behind particles which producea color equivalent to the color of the impinging light source. Thus, forexample, red light exposure causes the cyan colored pigment to migratethereby leaving behind the magenta and yellow pigments which combine toproduce red in the final image. In the same manner, blue and green colorlight is reproduced by removal of yellow and magenta pigmentrespectively and, or course, when white light impinges upon the mix allpigment migrate leaving behind the color of the white or transparentsubstrate. No exposure leaves behind all pigments which combine toproduce a black image. It should be recognized that this is an idealtechnique of subtractive color imaging in that the particle's colorcomponent performs a dual function in that it acts both as the finalimage colorant and the photosensitive medium of the system. Accordingly,the system represents virtually the ultimate in eliminating thecomplexity of prior art methods of subtractive color imaging.

It is desirable to use pigment particles which are relatively small insize because smaller particles produce better and more stable pigmentdispersions in the liquid carrier and in addition are capable ofproducing images of greater covering power and higher resolution thanwould be possible with particles of larger sizes. Even where thepigments are not commercially available in small particle sizes may bereduced by conventional techniques such as ball milling or the like.

The electrophoretic imaging particles when suspended in the liquidcarrier generally take on a net electrostatic charge such that they maybe attracted towards one of the electrodes in the system depending uponthe polarity of the charge with respect to that of the respectiveelectrode. Thus, the particles in the system generally are notrestricted to taking on only one polarity of charge but instead theparticles may be attracted to both electrodes. Some of the particles inthe suspension initially move towards the injecting electrode whileothers move towards the blocking electrode and the effects of imagewise,exposure-induced migration is superimposed upon this particle migration.Thus, the apparent bipolarity of these suspensions causes a portion ofthe suspended particles to be removed from the system along with thoseparticles which normally respond to the imagewise exposure. The effectof subtraction of those particles together with the radiationrespondingparticles substantially effects the quality of the image formed on thesurface to which the particles have migrated so as to make it unuseablein its present fonn. By electrophoretically depositing the imagingsuspension in the first instance in the absence of exposure radiationthe image degradating pigment particles are selectively abstracted fromthe suspension so that they no longer present in the system to effectthe resulting image produced on the imaging roller during the exposurephase of the process.

Any suitable different colored photosensitive pigment particles havingthe desired spectral responses such as disclosed in US. Pat. No.3,384,488 may be used to form the pigment mix in the carrier liquid forcolor imaging. The photosensitive pigment may, for example, be polymericin nature. The percentage of pigment in the insulating liquid carrier isnot considered critical; however, for reference purposes, it is notedthat from about 2 to about 10 percent pigment by weight has been foundto produce desirable and acceptable results.

As previously stated, once the particle image is formed it may be fixedto the respective electrode such as by spraying a binder onto thesurface, by laminating an overlay over the imaged surface or byincluding a binder in the liquid suspension medium. Generally, it willbe found preferable to transfer the image from the electrode and fix iton a secondary surface so that the electrode may be reused. Such atransfer step may be carried out by an adhesive pickoff technique suchas with adhesive tape or preferably by electrostatic field transfer.However, more desirable yet is to utilize a receptive sleevelikematerial on the blocking electrode which may simply be removed from therespective electrode following imaging.

Although various electrode spacings may be employed, spacings of lessthan about 1 mil and extending down to where the electrodes aresubstantially in virtual contact resulting from being pressed togetherare preferred. The latter condition constitutes a particularly preferredform of the invention in that there is produced the optimum imageresolution and in view of the present invention the most noticeableimprovement in color separation. This noted improvement is believed totake place as result of the high field strength across the suspensionduring imaging.

PREFERRED EMBODIMENTS To further define the specifics of the presentinvention the following examples are intended to illustrate and notlimit the subject matter of the present invention. Parts and percentagesare by weight unless otherwise indicated.

All of the following examples are carried out in an apparatus of thegeneral type illustrated in the figures with the imaging mixelectrophoretically coated on a NESA glass substrate. The NESA glasssurface is connected in series with a switch, a potential source, andthe conductive center of the roller electrodes utilized. The roller isapproximately 2 A inches in diameter and, except where specified, ismoved across the plate surface at about 3 inches per second. The NESAplate employed is roughly 3 inches square and the imaging mix is exposedthrough the plate with a light intensity of about 100 foot candles.Exposure is made with a 3,200 K. lamp through a Kodacolor negative whichis placed between the white light source and the NESA glass substrate.

EXAMPLE I An imaging suspension comprising equal amounts of Watchung RedB, a barium salt of l-(4-methyl-5'-chloro-2'- sulfonic acid)azobenzene-2-hydroxy-3-naphthoic acid, C.l. No. 15,865, available fromE. I. duPont de Nemours & Co., Monolite Fast Blue 6.8., the alpha formof metal-free phthalocyanine, C.l. No. 74,l00, available from ArnoldHoffman Company and a yellow pigment Algol Yellow G.C., l,2,5,6-di(C,C'-diphenyl)-thiazole anthraquinone, C.l. No. 67 ,300, availablefrom General Dyestuffs, in Sohio Solvent 3,440, a petroleum fractionavailable from Standard Oil of Ohio, is prepared with the total pigmentconstituting about 8 percent by weight of the suspension. These pigmentsare magenta, cyan, and yellow, respectively. The resulting mixture iscoated on the surface of a first roller electrode and with a negative3,000 volts applied to the roller, the trimix is electrophoreticallydeposited on a NESA glass substrate, in the absence of light. The coatedNESA glass is then exposed as discussed above such that a colored imageis projected onto the trimix as a second roller electrode moves acrossthe surface of the NESA glass. A cellophane paper blocking electrode isemployed and the roller is held at a negative potential of about 1,000volts with respect to the NESA glass substrate. The image picked up onthe paper surface consists of the complementary colors of the input, andan optically positive color image is obtained. The cellophane paper isthen removed for the imaging roller.

EXAMPLE II An imaging suspension comprising equal amounts of Bonadur RedB, l-(4'-chloro-5'-ethyl-2' sulfonic acid)azobenzene-2-hydroxy-3-napthoic acid, available from American Cyanamide,Monolite Fast Blue (1.8., the alpha form of metal-free phthalocyanine,C.l. No. 74,100, available from Arnold Hoffman Co. and a proprietaryyellow pigment N-2"- pyridyl-8,l3-dioxodinaphtoho-(2,1-6; 2', 3'-d)furan-o-carboxamide, more completely defined in U.S. Pat. applicationNo. 42l,28l filed Dec. 28, 1964 and having a common assignee, now U.S.Pat. No. 3,447,922, in Sohio Solvent 3,440 is prepared with the totalpigment constituting about 8 percent by weight of the suspension. Thesepigments are magenta, cyan and yellow respectively. The resultingmixture is coated on the surface of a first roller electrode and with anegative 2,000 volts applied to the roller, the trimix iselectrophoretically deposited on a NESA glass substrate in the absenceof light. The coated NESA glass is then exposed as discussed above suchthat a colored image is projected onto the trimix as a second rollerelectrode moves across the surface of the NESA glass. A baryta paperblocking electrode is employed and this roller is held at a negativepotential of about 1,200 volts, with respect to the NESA glasssubstrate. The image picked up on the baryta surface consists of thecomplementary colors of the input and an optically positive color imageis obtained. The bartya paper is then removed from the imaging roller.

EXAMPLE Ill The process of example II is repeated with the exceptionthat the second imaging roller is moved across the plate surface atabout 2 inches per second. Results similar to those obtained in examplell are demonstrated.

EXAMPLE IV The process of example I is repeated with the exception thatthe yellow pigment utilized in example ll is substituted for the AlgolYellow of example I. The remaining steps of the process are the same.Similar results are obtained.

EXAMPLE V The process of example IV is repeated with the exception thatthe voltage on the first roller electrode is a negative 3,000 volts andthe voltage on the second roller electrode is a negative 750 volts.

EXAMPLE VI The process of example I is repeated with the exception ofthe substitution of Tedlar for the cellophane paper on the blockingelectrode. The remainder of the process remains the same. The colorpositive print is reproduced on the separable Tedlar substrate.

EXAMPLE Vll An imaging suspension comprising a metal-free phthalocyaninepigment, Monolite Fast Blue (3.8., is prepared, 7 parts by weight of thephotosensitive particles being dispersed in Sohio Odorless solvent3,440. The pigment suspension is electrophoretically coated on a NESAglass substrate at a negative 2,500 volts and then exposed such that animage is projected onto the mix as a second imaging roller moves acrossthe surface. A cellophane paper blocking electrode is utilized'as thesecond roller with a negative potential of about 2,000 volts beingdeveloped. In this example with the use of only one pigment in theimaging suspension a direct positive monochrome image is produced.

Although the present examples are specific in terms of conditions andmaterials used any of the above-mentioned materials may be substitutedwhen applicable with similar results being obtained. In addition to thesteps used in the process of the present invention other steps ormodifications may be used if desirable. For example, the system couldalso be adapted to direct positive to negative imaging if desired. Inaddition, other materials may be incorporated in the imaging suspensionand the other facets of the invention which will enhance, synergize orotherwise desirably effect the properties therein desired. For example,various sensitizers may be utilized in conjunction with the imagingsuspension.

Anyone skilled in the art will have other modifications occur to thembased on the teachings of the present invention. These modifications areintended to be encompassed within the scope of this invention.

What is claimed is:

l. A method of photoelectrophoretic imaging comprisingelectrophoretically depositing an imaging suspension on the surface of asubstantially transparent electrode, said suspension comprising aplurality of finely divided particles in a carrier liquid each of saidparticles comprising an electrically photosensitive pigment, saidpigment being both the primary electrically photosensitive ingredientand the primary colorant for said particle, introducing at least oneadditional electrode, subjecting said suspension to an applied electricfield between said electrodes, exposing said suspension to an imagethrough said transparent electrode with a source of activatingelectromagnetic radiation whereby an image is formed.

2. The process as disclosed in claim 1 wherein said electrodes arespaced in virtual contact during exposure.

3. The process as disclosed in claim 1 further including the step ofbringing said electrodes into virtual contact while exposing saidsuspension.

4. A method of photoelectrophoretic imaging comprisingelectrophoretically depositing an imaging suspension on the surface of asubstantially transparent electrode, said suspension comprising aplurality of at least two differently colored finely divided particlesin a carrier liquid, each of said particles comprising an electricallyphotosensitive pigment whose principal light absorption bandsubstantially coincides with its principal photosensitive response,introducing at least one additional electrode, subjecting saidsuspension to an applied electric field between said electrodes,exposing said suspension to an image through said transparent electrodewith a source of activating electromagnetic radiation whereby an imageis formed.

5 An electrophoretic imaging process comprising electrophoreticallydepositing a layer of an imaging suspension on the surface of asubstantially transparent electrode, said suspension comprising aplurality of finely divided particles in a substantially insulatingcarrier liquid each of said particles comprising an electricallyphotosensitive pigment which is both the primary electricallyphotosensitive ingredient and the primary colorant for said particle,introducing at least one additional electrode into the system,subjecting said layer of suspension to an applied electrical fieldbetween said electrodes while substantially simultaneously exposing saidsuspension to a light image through said transparent electrode therebyproducing complementary pigment images on the surface of each of saidelectrodes.

6. The process as disclosed in claim 5 further including the step ofbringing said electrodes together to a spacing of up to about 1 milwhile exposing said suspension.

7. An electrophoretic imaging process comprising electrophoreticallydepositing a layer of imaging suspension on the surface of asubstantially transparent electrode, said suspension comprising aplurality of at least two differently colored finely divided particlesin a carrier liquid each of said particles comprising an electricallyphotosensitive pigment the principal light absorption band of whichsubstantially coincides with its principal photosensitive response,introducing at least one additional electrode into the system,subjecting said layer of suspension to a light image through saidtransparent electrode, subjecting said layer of suspension to an appliedelectric field between said electrode thereby producing complementarypigment images on the surfaces of each of said electrodes.

8. An electrophoretic imaging process comprising electrophoreticallydepositing a layer of an imaging suspension on the surface of asubstantially transparent electrode, said suspension comprising aplurality of finely divided particles in a substantially insulatingcarrier liquid each of said Eparticles comprising an electricallyphotosensitive pigment which is both the primary electricallyphotosensitive ingredient and the primary colorant for said particle,said suspension including cyan colored particles principallyphotosensitive to red light, magenta colored particles principallyphotosensitive to green light and yellow colored particles which areprincipally photosensitive to blue light, providing at least oneadditional electrode for the system, subjecting said layer of suspensionto an applied electrical field between said electrodes, exposing saidsuspension to a light image through said transparent electrode whilesustaining said electrical field whereby an image is formed.

9. An electrophoretic imaging process comprising the steps of:

electrophoretically depositing a layer of an imaging suspension on thesurface of a first electrode, said suspension comprising a plurality ofelectrically photosensitive particles in an electrically insulatingcarrier, contacting the suspension on the surface of the electrode withat least another electrode,

exposing said suspension to an image with activating electromagneticradiation during contact with at least another electrode,

applying an electrical field between said electrodes during imaging.

10. The process of claim 9 wherein the electrophoretic deposition isaccomplished by depositing a layer of suspension on the first electrodeand placing a removal electrode across the suspension to remove unwantedparticles and applying an electrical field across the suspension betweenthe first electrode and removal electrode whereby particles within thesuspension are selectively removed from the first electrode.

1]. The process of claim 9 wherein the electrophoretic deposition isaccomplished by passing a suspension depositing electrode across thefirst electrode and applying an electrical field across the suspensionwhereby the suspension particles are selectively depositedelectrophoretically on the first electrode.

12. The process of claim 9 wherein the electrophoretic depositionincludes applying an electrical field across the suspension.

13. The process of claim 12 wherein the electrical field applied duringelectrophoretic deposition is of the same polarity as the polarity ofthe field applied during imaging.

14. The process of claim 13 wherein the electrical fields are such thatthe surface of the first electrode upon which the layer iselectrophoretically deposited is positive relative to electrodesinterfacing therewith during electrophoretic deposition and imaging.

2. The process as disclosed in claim 1 wherein said electrodes arespaced in virtual contact during exposure.
 3. The process as disclosedin claim 1 further including the step of bringing said electrodes intovirtual contact while exposing said suspension.
 4. A method ofphotoelectrophoretic imaging comprising electrophoretically depositingan imaging suspension on the surface of a substantially transparentelectrode, said suspension comprising a plurality of at least twodifferently colored finely divided particles in a carrier liquid, eachof said particles comprising an electrically photosensitive pigmentwhose principal light absorption band substantially coincides with itsprincipal photosensitive response, introducing at least one additionalelectrode, subjecting said suspension to an applied electric fieldbetween said electrodes, exposing said suspension to an image throughsaid transparent electrode with a source of activating electromagneticradiation whereby an image is formed. 5 An electrophoretic imagingprocess comprising electrophoretically depositing a layer of an imagingsuspension on the surface of a substantially transparent electrode, saidsuspension comprising a plurality of finely divided particles in asubstantially insulating carrier liquid each of said particlescomprising an electrically photosensitive pigment which is both theprimary electrically photosensitive ingredient and the primary colorantfor said particle, introducing at least one additional electrode intothe system, suBjecting said layer of suspension to an applied electricalfield between said electrodes while substantially simultaneouslyexposing said suspension to a light image through said transparentelectrode thereby producing complementary pigment images on the surfaceof each of said electrodes.
 6. The process as disclosed in claim 5further including the step of bringing said electrodes together to aspacing of up to about 1 mil while exposing said suspension.
 7. Anelectrophoretic imaging process comprising electrophoreticallydepositing a layer of imaging suspension on the surface of asubstantially transparent electrode, said suspension comprising aplurality of at least two differently colored finely divided particlesin a carrier liquid each of said particles comprising an electricallyphotosensitive pigment the principal light absorption band of whichsubstantially coincides with its principal photosensitive response,introducing at least one additional electrode into the system,subjecting said layer of suspension to a light image through saidtransparent electrode, subjecting said layer of suspension to an appliedelectric field between said electrode thereby producing complementarypigment images on the surfaces of each of said electrodes.
 8. Anelectrophoretic imaging process comprising electrophoreticallydepositing a layer of an imaging suspension on the surface of asubstantially transparent electrode, said suspension comprising aplurality of finely divided particles in a substantially insulatingcarrier liquid each of said particles comprising an electricallyphotosensitive pigment which is both the primary electricallyphotosensitive ingredient and the primary colorant for said particle,said suspension including cyan colored particles principallyphotosensitive to red light, magenta colored particles principallyphotosensitive to green light and yellow colored particles which areprincipally photosensitive to blue light, providing at least oneadditional electrode for the system, subjecting said layer of suspensionto an applied electrical field between said electrodes, exposing saidsuspension to a light image through said transparent electrode whilesustaining said electrical field whereby an image is formed.
 9. Anelectrophoretic imaging process comprising the steps of:electrophoretically depositing a layer of an imaging suspension on thesurface of a first electrode, said suspension comprising a plurality ofelectrically photosensitive particles in an electrically insulatingcarrier, contacting the suspension on the surface of the electrode withat least another electrode, exposing said suspension to an image withactivating electromagnetic radiation during contact with at leastanother electrode, applying an electrical field between said electrodesduring imaging.
 10. The process of claim 9 wherein the electrophoreticdeposition is accomplished by depositing a layer of suspension on thefirst electrode and placing a removal electrode across the suspension toremove unwanted particles and applying an electrical field across thesuspension between the first electrode and removal electrode wherebyparticles within the suspension are selectively removed from the firstelectrode.
 11. The process of claim 9 wherein the electrophoreticdeposition is accomplished by passing a suspension depositing electrodeacross the first electrode and applying an electrical field across thesuspension whereby the suspension particles are selectively depositedelectrophoretically on the first electrode.
 12. The process of claim 9wherein the electrophoretic deposition includes applying an electricalfield across the suspension.
 13. The process of claim 12 wherein theelectrical field applied during electrophoretic deposition is of thesame polarity as the polarity of the field applied during imaging. 14.The process of claim 13 wherein the electrical fields are such that thesurface of the first electrode upon whiCh the layer iselectrophoretically deposited is positive relative to electrodesinterfacing therewith during electrophoretic deposition and imaging.